Radiative Spectra from Disk Corona and Inner Hot Flow in Black Hole X-ray Binaries

Abstract

To understand the origin of hard X-ray emissions from black hole X-ray binaries during their low/hard states, we calculate the X-ray spectra of black-hole accretion flow for the following three configurations of hot and cool media: (a) an inner hot flow and a cool outer disk (inner hot flow model), (b) a cool disk sandwiched by disk coronae (disk corona model), and (c) the combination of those two (hybrid model). The basic features we require for successful models are (i) significant hard X-ray emission whose luminosity exceeds that of soft X-rays, (ii) high hard X-ray luminosities in the range of (0.4 - 30) times 10^{37} erg s^{-1}, and (iii) the existence of two power-law components in the hard X-ray band with the photon indices of Gamma_s ~ 2 > Gamma_h, where Gamma_s and Gamma_h are the photon indices of the softer (<10 keV) and the harder (>10 keV) power-law components, respectively. Contribution by non-thermal electrons nor time-dependent evolution are not considered. We find that Models (a) and (b) can be ruled out, since the spectra are always dominated by the soft component, and since only one power-law component, at most, can be reproduced. Only Model (c) can account for sufficiently strong hard X-ray emissions, as well as the existence of the two power-law components, for a large ratio of the accretion rate in the corona to that in the thin disk. The outer disk corona (where the Compton y-parameter is smaller, y < 1) produces the softer power-law component with photon index of Gamma_s ~ 2, whereas the inner hot flow (where y gtrsim 1) generates the harder component with Gamma_h < 2. This model can also account for the observed relationship between the photon index and the reflection fraction.

13 pages, 13 figures, accepted for publication in PASJ

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• On the determination of the spin of the black hole in Cyg X-1 from X-ray reflection spectra
• Modelling variability in black hole binaries: linking simulations to observations
• The soft component and the iron line as signatures of the disc inner radius in Galactic black hole binaries
• Detection of Coronal Magnetic Activity in Nearby Active Supermassive Black Holes
• Evidence for a Cool Disk and Inhomogeneous Coronae from Wide-band Temporal Spectroscopy of Cygnus X-1 with Suzaku
• The Emission from Inner Disk and Corona in the Low and Intermediate Spectral States of Black Hole X-ray Binaries
• Effects of adiabatic index on the sonic surface and time variability of low angular momentum accretion flows
• The Spectral Features of Disk and Corona with Mass Evaporation in the Low/Hard State
• Revisiting the structure and spectrum of the magnetic-reconnection-heated corona in luminous AGNs
• Proton Acceleration by Collisionless Shocks in Supermassive Black Hole Coronae: Implications for High-Energy Neutrinos